Cells having cavities, such as vapor cells, may be used in different industrial and/or scientific applications. Some applications in which these types of cells may be utilized include physics (e.g., experimental physics), biotechnology, space research or lab on a chip applications.
Vapor cells containing or filled with a reactive gas may be used as a resonator element in atomic clocks to provide a time standard. In atomic clock applications, hyperfine structure transition of gaseous alkali metals such as cesium or rubidium provide a high precision frequency or time standard.
While known methods of inducing hyperfine structure transition include using microwave radiation, another method of inducing hyperfine structure transition, known as coherent population trapping (CPT), uses optically induced resonance instead. In contrast to inducing hyperfine structure transition using microwave radiation, CPT enables significant miniaturization of reactive gas-filled vapor cells and its corresponding components, thereby enabling the associated atomic clocks to be built smaller (e.g., miniaturized) as well. Miniaturized atomic clocks are popular especially for non-stationary uses even though these atomic clocks may have relatively lower accuracy.
Miniaturized atomic clocks and their corresponding miniaturized vapor cells may be utilized in non-stationary equipment applications for satellite navigation systems (e.g., GPS, GLONAA, or Galileo). As compared to other approaches that require signals from four satellites to determine a position of an object, implementing satellite navigation systems with miniaturized atomic clocks enable a relatively more accurate position of an object to be determined based on signals from only three satellites.
In other applications, miniaturized atomic clocks and their corresponding miniaturized vapor cells may be used to enable the synchronization of signals in communication networks or cryptography keys. Additionally, vapor cells filled with reactive gas may be utilized in high-precision magnetic field sensors or rotational speed sensors of nuclear magnetic resonance gyroscopes (NMRG).
Macroscopic vapor cells may be manufactured using fine machining techniques such as, individually filling glass capsules with a desired material and then carefully closing the glass capsules using glass welding or glassblowing techniques. Hermetic bonding or sealing of silicon and/or glass may be utilized during the manufacturing of microscopic vapor cells. However, similar to the manufacturing process of macroscopic vapor cells in which these vapor cells are filled prior to closing, microscopic vapor cells must be filled with the desired material prior to hermetic sealing.
U.S. patent publication number 2006/0022761 relates to a process of manufacturing vapor cells (e.g., vapor gas cells) filled with cesium. In the process described, using processes known from semiconductor technology, a penetrating hole is etched in a silicon wafer that corresponds to the interior dimensions of the vapor cell. The silicon wafer is then connected to a glass wafer using anodic bonding to close the penetrating hole. In a cavity created during etching, fluid cesium is then introduced into a nitrogen or argon atmosphere. Thereafter, the cavity is hermetically sealed by another glass wafer using anodic bonding. The gas enclosed in the cavity (e.g., argon or nitrogen) acts as a buffer gas during the use of the vapor cell.
U.S. Pat. No. 6,900,702 B2 relates to a process of introducing rubidium into a vapor cell that is a component of a frequency standard based on silicon wafers. Once the rubidium is introduced into the vapor cell, the vapor cell is sealed.
Both U.S. patent publication number 2006/0022761 and U.S. Pat. No. 6,900,702 B2 also describe changing a vapor pressure within the vapor cells by heating the sealed vapor cells and the alkali metals contained therein with a laser or other heating element.
The publication, Li-Anne Liew et al. in Appl. Phys. Lett. Vol. 84 no. 14 dated Apr. 5, 2004, describes an alternative approach of filling vapor cells (e.g., vapor pressure cells) using wafers. Instead of filling the vapor cell with pure cesium, cesium chloride and barium azide are introduced into the vapor cell and then the vapor cell is sealed. After sealing, a chemical reaction between the cesium chloride and the barium azide may be initiated that yields atomic cesium.
The processes described above have disadvantages that prevent cost effective production. For example, alkali metals such as cesium and rubidium are very reactive with, for example, water vapor and oxygen, requiring a protected atmosphere when handling. Therefore, when filling vapor cells with these alkali metals, special systems and significant care must be taken to ensure safety.
As mentioned above, barium azide may be used in processes based on reactive gas formation by chemical reaction taking place within the vapor cell. However, azides and particularly barium azide are highly dangerous substances that may be used as explosives. Because azides cannot be handled or transported in Europe, these substances are essentially unavailable.
Reference is also made to patent publications DE 692 05 307 T2 and DE 696 29 483 T2.